JPH11240799A - Single crystal substrate and manufacturing method thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a single crystal substrate excellent in surface roughness, having extremely small face sagging and smooth surface by keeping average roughness in the center line of a substrate surface, face sagging of the substrate and sagging of the substrate to thickness direction to a specific value. SOLUTION: In this substrate, average roughness in the center line of a substrate surface is set to <=0.3 nm and face sagging of the substrate is set to a value within 20 μm from the periphery of the substrate and sagging of the substrate to thickness direction is set to 1 μm. Such substrate is obtained by cutting single crystal ingot to afford a plate-like wafer, polishing the wafer to a desired thickness by both-face lap, attaching a formed body for polishing the wafer until, a desired thickness by both-face lap, attaching a formed body for polishing consisting mainly of silica (silicon dioxide) and having 0.2-1.5 g/cm<3> bulk density, 10-400 m<2> /g BET specific surface area and 0.001-0.5 μm average particle diameter to a polishing apparatus to afford a surface plate, pressing the wafer to the surface plate for polishing and polishing the wafer. The single crystal substrate includes especially preferably an oxide substrate of lithium borate or lithium tantalate.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a single crystal substrate such as lithium tantalate or lithium borate used for optical or surface acoustic wave filters such as optical waveguides, and a method for producing the same.

[0002]

2. Description of the Related Art In the field of mobile communication such as automobile telephones and cordless phones, single crystal substrates such as lithium tantalate and lithium borate are used as substrates for surface acoustic wave devices. This substrate is conventionally manufactured as follows. That is, for example, a single-crystal ingot obtained by the Choklaskie method or the like is cut using a cutting machine or a wire saw for an inner peripheral blade to obtain a plate-shaped wafer, and both sides of the obtained wafer have a predetermined thickness. Wrap until then, for example, the wafer is fixed to the upper platen of a single-side polishing apparatus, and mirror-polished using a polishing pad containing urethane or the like and a polishing liquid containing colloidal silica as free abrasive grains, and after polishing is completed. It is obtained by removing the polished wafer from the upper platen and washing it with an organic solvent.

The important points in the characteristics of a single crystal substrate used for an optical or surface acoustic wave filter such as an optical waveguide are as follows.

[0004] 1) When irradiating light to a substrate, it is extremely important in optical applications to minimize irregular reflection of light on the surface of the substrate. It has become.

2) When the substrate is put to practical use, the flat portion of the polished substrate is cut into a required size and used, so that it is more economically advantageous to have more flat portions. Therefore, in the polishing step in the substrate manufacturing process, the peripheral portion of the polished substrate is not thinner than its center portion, that is, its flatness is ensured by not causing a so-called sagging phenomenon. Many areas can be put to practical use.

3) For example, when a pattern is formed on the surface of a substrate by photolithography or the like, a flat surface can form a pattern more accurately, and a finer pattern can be formed. Surface is flat.

However, in the conventional single crystal substrate, the center line average roughness of the polished surface of the substrate is at most 0.4 nm, and the edge of the polished surface of the substrate becomes thinner than the center.
A so-called surface sagging phenomenon has occurred, which may extend from the periphery of the substrate to the inside of the substrate to a range of about 500 μm, and usually sagging in the thickness direction of the substrate of 50 μm or more. In addition, in the case of the conventional manufacturing method, the center line average roughness of the polished surface of the single crystal substrate is at most 0.4 nm, and the above-described surface drooping phenomenon has occurred, and the center line average roughness can be accurately determined. In order to perform the polishing, the polishing time is further required, so that the sagging phenomenon is promoted.

Also, Machines and Tools, Vol. 15, No. 8, 23-
On page 28 (August 1994), there is described a method using fixed abrasive polishing obtained by fixing free abrasive grains to a support instead of conventional polishing using free abrasive grains. In the case of polishing with fixed abrasive grains, scratches are likely to occur and the finished surface roughness is poor, and it is difficult to control the production of these fixed abrasive grains, and to obtain fixed abrasive grains having a uniform structure Is a major issue.

That is, in either of the conventional method using loose abrasives and the method using fixed abrasives, the surface roughness of the substrate is made to be high precision, and the surface of the substrate is hardly sagged at the peripheral portion. It was difficult to do.

Furthermore, in the conventional method of polishing a substrate surface using a polishing pad such as urethane and a polishing liquid containing colloidal silica as free abrasive grains, it is also difficult to make the substrate surface flat by polishing. Was.

[0011]

In the case of a single crystal substrate of lithium tantalate or lithium borate having a center line average roughness polished by such a conventional method, for example, further fine processing in an element forming step is required. Not done,
Making the center line average roughness of the substrate even more precise is an important issue, and by making the center line average roughness of the substrate even more precise, finer processing is possible in the element fabrication process. The characteristics can be improved. Also,
In the case of a single crystal substrate having surface droop, the yield when processing the substrate into a device chip is low, and suppression of the surface droop phenomenon has been an important issue. Furthermore, making the surface of the substrate flat has been an important issue when applying an appropriate pattern to the substrate surface by photolithography or the like.

That is, an object of the present invention is to provide a single crystal substrate in which the surface of the substrate is a smooth surface with high precision, the surface droop caused by such surface dripping is small, and the substrate surface is flat. It is to provide a manufacturing method thereof.

[0013]

Means for Solving the Problems As a result of intensive studies to solve the above-mentioned problems, the present inventors cut a single crystal ingot to obtain a plate-like wafer, and wrapped the wafer with a desired double-sided wrap. In a method of manufacturing a single-crystal substrate by polishing to a thickness after polishing to a thickness, the mirror-polishing treatment mainly comprises silica (silicon dioxide) and a bulk density of 0.2 to 1.5 g / cm ^3. , The BET specific surface area is 10 to 400 m ² / g, and the average particle diameter is 0.0
The following findings were found by mounting a molded body for polishing having a size of from 01 to 0.5 μm to a polishing apparatus to form a polishing surface plate, and pressing and polishing a wafer against the polishing surface plate.

1) In polishing a single crystal substrate, which is a hard material, a polishing compact obtained by molding silica fine particles is used. By devising a method, the surface properties of the molded body for polishing can be controlled, and as a result, the surface of the polished substrate can be polished with extremely high precision even in a relatively short time, and a smooth surface can be obtained. can do.

2) The molded article for polishing is an integral molded article,
Since the physical properties are also hard, the polishing molded body is polished to the corners of the substrate by the same force during the polishing, so that the so-called surface dripping phenomenon can be significantly suppressed in the polished substrate.

3) When a plate-shaped molded article is used as the molded article for polishing, a force is uniformly applied to the entire polished surface of the substrate during polishing, and therefore, the polished substrate has a flat surface. To have.

In particular, it has been found that by setting the surface properties of the molded article for polishing within a certain range, the shape of the substrate can be made constant, and a material having excellent surface smoothness can be obtained. Reached.

Hereinafter, the present invention will be described in detail.

Examples of the single crystal substrate of the present invention include oxide substrates such as lithium borate, lithium tantalate and lithium niobate, silicon wafers, compound substrates such as gallium arsenide and gallium phosphide, and quartz. Among them, oxide substrates such as lithium borate, lithium tantalate and lithium niobate and silicon wafers are preferable in terms of excellent surface accuracy, and oxide substrates of lithium borate and lithium tantalate are more preferable.

The single crystal substrate of the present invention has a center line average roughness of the substrate surface of 0.3 nm or less, preferably 0.25 nm or less, and the surface drooping of the substrate from the periphery of the substrate as shown in FIG. It is within the range of 20 μm or less, and the droop in the thickness direction of the substrate is 1 μm or less.

As described above, a single crystal substrate having a small center line average roughness and a small surface sag is difficult to manufacture, and therefore, no practical substrate has been obtained. And lithium tantalate single crystal substrates have a center line average roughness of at most about 0.4 nm, and the surface roughness may range within 500 μm from the periphery of the substrate, and is usually 50 μm or more in the thickness direction of the substrate. The single crystal substrate of the present invention is completely new, and is very poor.

Here, the center line average roughness of the single crystal substrate is defined in JIS-B-0601, and the measurement is usually carried out using a stylus type surface roughness as described in JIS-B-0651. It can be measured by a stylus-type surface roughness meter Taristep, for example.
This device is one of the devices that can measure the surface roughness very accurately, and the measurement limit value of the center line average roughness is 0.1 nm.
It is. However, even if the surface roughness of a polished single crystal substrate is measured using this apparatus, it is very close to the measurement limit of the center line average roughness, and it is practically extremely difficult to compare the degree of surface roughness of the substrate. It is difficult.

Then, the present inventors measured the center line average roughness using an atomic force microscope (AFM) capable of measuring the center line average roughness more accurately, and found that a single crystal substrate having an excellent center line average roughness was found. Reached. Therefore, the value of the center line average roughness in the present specification is a value of the center line average roughness measured by an atomic force microscope (AFM) or a measurement method capable of measuring the center line average roughness with the same accuracy as this. It is.

It has such excellent center line average roughness,
The method for manufacturing a single crystal substrate with extremely small surface droop is not particularly limited as long as a substrate having the above characteristics can be manufactured, and can be easily manufactured by the following manufacturing method.

That is, usually, a single crystal substrate, for example, a lithium borate single crystal substrate, grows a single crystal of lithium borate to obtain an ingot, and then cuts it to obtain a plate-shaped wafer. This wafer is polished to a desired thickness with a double-sided lap, and the obtained wafer is mirror-polished (usually called “polishing”).

Here, in the case of mirror-polishing a wafer wrapped on both sides, as described below, there is a method of manufacturing by pressing the wafer against a molded body for polishing having a specific property and sliding the wafer. A simple and single-crystal substrate exhibiting excellent performance can be obtained. further,
Incorporating the molded body for polishing as a polishing surface plate, and mounting the polishing surface plate on a polishing apparatus to perform polishing, thereby improving work efficiency. Therefore, such a polishing surface plate and a polishing device are used. Embodiments are preferred. The polishing platen is not particularly limited as long as it is a commonly used polishing plate, and the polishing apparatus is capable of applying an appropriate pressure to the wafer when polishing the wafer to a mirror surface and performing a rubbing motion during polishing. I just need.

The polishing compact used for mirror polishing the wafer is mainly composed of silica (silicon dioxide), has a bulk density of 0.2 to 1.5 g / cm ³ ,
It is preferable that the ET specific surface area is 10 to 400 m ² / g and the average particle diameter is 0.001 to 0.5 μm. Is polished by pressing the surface of the molded body for polishing and the surface of the substrate while circulating, and performing a sliding motion. At this time, the polishing liquid may or may not contain free abrasive grains. However, the use of free abrasive grains usually results in a higher polishing rate and easy production.

Here, the physical properties and composition of the molded article for polishing are as described below.

That is, silica mainly having a silica component of 90% by weight or more of the total amount is preferably used, and examples thereof include dry-process silica and wet-process silica. Further, for example, a binder other than silica is not included. Here, the silica component is 90% by weight or more when the total amount of the silica component, impurities, and ignition loss (Ig. Loss) after heating the silica powder at 105 ° C. for 2 hours is the weight% of the silica component. It is. Therefore, considering the loss of burning, the silica raw material powder used in the present invention has a silica component of substantially 97% by weight or more.

The bulk density of the molded body for polishing is 0.2
It is preferably in the range of to 1.5 g / cm ^3, more preferably from 0.4 to 0.9 g / cm ^3. If the bulk density is less than 0.2 g / cm ³ , production becomes difficult, and 1.5
If it exceeds g / cm ³ , it becomes difficult to obtain excellent surface roughness, which is not preferable.

The BET specific surface area of the abrasive compact is as follows:
Is preferably in the range of 10 to 400 m ² / g, more preferably from 10 to 200 m ² / g. Production and BET specific surface area is greater than 400m ² / g, it becomes difficult, 10m ²
/ G, it becomes difficult to obtain excellent surface roughness.

The average particle size of the molded article for polishing is set to 0.1.
The range is preferably from 001 to 0.5 μm, more preferably from 0.01 to 0.3 μm. The average particle size is 0.
If it is less than 001 μm, the production becomes difficult, and if it exceeds 0.5 μm, it becomes difficult to obtain excellent surface roughness.

The abrasive compact having the above-mentioned characteristics can be obtained by processing a silica compact obtained by molding a silica fine powder. Examples of the processing method include a method using heat degreasing, heat baking, machining, and the like, but are not particularly limited as long as the processing method can provide a strength that can be used in a polishing operation as a molded body for polishing.

Next, it is possible to manufacture a single crystal substrate by using the molded body for polishing as a polishing platen. For example, there is a method in which a molded body for polishing is fixed to a rotating platen of a polishing apparatus, and the surface of a substrate is polished.

The conditions for polishing in the production of a single crystal substrate using such a molded article for polishing are not particularly limited as long as a single crystal substrate having the above characteristics can be produced. For example, colloidal silica or the like is usually used as an abrasive. In the conventional industrial practice, the concentration is used at a high concentration containing, for example, 20 to 30% by weight of silicon dioxide. In the present invention, the concentration is selected from an arbitrary range up to this concentration. Polishing can be sufficiently performed at about 20% by weight. Further, since the abrasive compact itself has a function as an abrasive, it can be manufactured without using free abrasive grains.

Thus, the single crystal substrate of the present invention can be obtained. The use thereof is excellent in surface accuracy, and is suitably used for optical applications such as optical waveguides or surface acoustic wave filters. .

In the case of a single crystal substrate of lithium borate, there are lithium borate having different ratios of boron, lithium and oxygen, for example, LiBO ₄ and Li ₂
There are B ₄ O _{7 and the} like, but in the present invention, some of them include all lithium borate existing as a single crystal.

A method for manufacturing a single crystal substrate using such a molded body for polishing is described in connection with a material other than a single crystal substrate made of an oxide of lithium tantalate or lithium borate, for example, an oxide substrate such as lithium niobate, silicon, or the like. It can be applied to wafers, compound substrates such as gallium arsenide and gallium phosphide, and other materials such as quartz, and even with those materials, unprecedented surface accuracy, that is, low center line average roughness, and excellent surface roughness Substrate material can be obtained. Furthermore, it is possible to obtain a substrate having higher surface accuracy than that obtained by a conventional method even for a polycrystalline substrate such as glass, metal, ferrite, alumina, aluminum nitride, and PZT. Conceivable.

In the conventional polishing method using a polishing cloth such as urethane, since the polishing cloth is formed of a material having a high elasticity, it is well suited to the surface of the material to be polished having irregularities. In the method of manufacturing a single crystal substrate using the molded body for polishing of the present invention, the hardness of the platen used during polishing is relatively hard compared to the polishing cloth, and the flatness and precision of the platen itself are reduced. It is considered that the surface accuracy of the material to be polished is also improved due to the influence. That is, it is presumed that a substrate material excellent in surface roughness and having a very small surface roughness on the substrate surface can be obtained.

However, such a presumption does not restrict the present invention at all.

[0041]

EXAMPLES Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited thereto.
In addition, each evaluation was implemented by the method shown below.

Bulk Density A flat sample having a size of 100 mm × 100 mm × 15 mm (thickness) was prepared and used as a sample. This sample was calculated from the weight measured with an electronic balance and the shape and dimensions measured with a micrometer.

[Average Particle Size] A part of the abrasive compact was subjected to scanning electron microscope ISI DS
Observed with -130 (manufactured by Akashi Seisakusho), and determined by the intercept method considering only the silica particle portion.

~ BET specific surface area ~ After grinding the compact for polishing, MONOSORB (QUA
(Antachrom Co., Ltd.) using the BET one-point method.

-Surface Roughness- To measure the degree of surface roughness of a single crystal substrate, the center line average roughness is generally defined in JIS-B-0601, and JIS-B-0651 is used for the measurement. Can be measured with a stylus-type surface roughness meter as described in (1), for example, with a stylus-type surface roughness meter Taristep. This device is one of the devices that can measure the surface roughness very accurately, and the measurement limit value of the center line average roughness is 0.
1 nm. Therefore, the surface roughness of the polished lithium tantalate and lithium borate single crystal substrates in Examples and Comparative Examples was measured using this apparatus. Table 1 shows the measurement conditions, and Table 2 shows the measurement results. The measurement results were the same for both the lithium tantalate single crystal substrate and the lithium borate single crystal substrate. Tally step (RANK for measurement)
TAYLOR HOPSON LTD. Made, Model: M
XX9-1650-XX 112/1037 396
A) was used. As shown in Table 2, even when the surface roughness of the polished lithium tantalate and lithium borate single crystal substrates measured using this apparatus is close to the measurement limit of the center line average roughness, the surface roughness It was practically very difficult to make a comparison of the degree.

[0046]

[Table 1]

[0047]

[Table 2]

Therefore, the surface roughness of the polished lithium tantalate and lithium borate substrates was measured using an atomic force microscope (AF).
M) Using SPI3600 (manufactured by SII), the repulsive force was measured by a contact mode. The measurement is 2μm × 2μ on lithium tantalate or lithium borate substrate
The range of m was arbitrarily measured for each of three regions, averaged, and evaluated by center line average roughness (Ra). Here, the end of polishing means a stage where a surface equivalent to a polished surface, which is generally referred to as a product in both the example and the comparative example, is obtained. This is a state where the finished surface is mirror-polished by about 10 μm.

The surface drooping phenomenon was observed using a scanning electron microscope (manufactured by Akashi Seisakusho, model: ISI DS-130), and the amount of surface drooping as shown in FIG. Defined and calculated. That is, assuming that the substrate is a flat surface and the non-planar portion at the peripheral edge of the substrate is first defined as a distance (distance A in FIG. 1) from the peripheral edge of the substrate surface to the inside, and The distance from the point where the tangent of (1) intersects the tangent to the surface of the substrate to the portion where the droop occurs (distance B in FIG. 1).

Example 1 A cylindrical test piece of a molded article for polishing having a diameter of 25 mm and a thickness of 5 mm having the physical properties shown in Table 3 was prepared, mounted on a rotating platen (360 mm in diameter) of a high-speed lens polishing apparatus, and molded. The body surface was flattened. Using a 3 inch diameter lithium tantalate substrate as the material to be polished, a commercially available colloidal silica (manufactured by Fujimi Incorporated, COMPOL 80) having a silica (silicon dioxide) content of 8% by weight.
The polishing liquid was dropped at a rate of 100 ml / min and circulated and used at a temperature of 12 ° C. and pH = 12). Table 4 shows the measurement results of the surface accuracy as the obtained results, and FIG. 2 shows a sketch of an atomic force microscope photograph of the surface of the lithium tantalate substrate obtained by polishing.

[0051]

[Table 3]

[0052]

[Table 4]

The peripheral edge of the polished lithium tantalate substrate surface was photographed using a scanning electron microscope according to the method described above, and the distance A and the distance B were measured.
They were about 7 μm and about 0.3 μm, respectively.

Comparative Example 1 A suede-type polishing pad (SURFIN 018-3, manufactured by Fujimi Incorporated) was attached to a rotating platen (360 mm in diameter) of a high-speed lens polishing apparatus, and the platen was polished on the platen at a rotation speed of 100 rpm. Under a predetermined processing pressure of the material, a commercially available colloidal silica (COMPOL80, manufactured by Fujimi Incorporated) was prepared to have a silica (silicon dioxide) content of 20% by weight using a lithium tantalate substrate as a material to be polished. Using a polishing liquid (liquid temperature: 25 ° C., pH = 12), polish the polishing liquid to 100 m
Polishing was performed while dropping and circulating at a rate of 1 / min. Table 4 shows the measurement results of the surface accuracy as the obtained results, and FIG.
2 shows a sketch of an atomic force microscope photograph of the surface of the lithium tantalate substrate obtained by polishing.

Further, the periphery of the polished lithium tantalate substrate surface was photographed using a scanning electron microscope according to the method described above, and the distance A and the distance B were measured.
They were about 200 μm and about 100 μm, respectively.

Example 2 A cylindrical test piece of a molded body for polishing having a diameter of 25 mm and a thickness of 5 mm having the physical properties shown in Table 3 was prepared, mounted on a rotating platen (360 mm in diameter) of a high-speed lens polishing apparatus, and molded. The body surface was flattened. Using a lithium borate substrate having a diameter of 3 inches as a material to be polished under a predetermined rotation speed of the platen of 100 rpm and a predetermined processing pressure of the material to be polished on the surface plate, commercially available colloidal silica (Fujimi Incorporated, COMPOL 80) was prepared to have a silica (silicon dioxide) content of 8% by weight (liquid temperature: 25 ° C.,
Using pH = 12), the polishing liquid was dropped at a rate of 100 ml / min and polished while being circulated. Table 5 shows the measurement results of the surface accuracy as the obtained results, and FIG. 4 shows a sketch of an atomic force microscope photograph of the surface of the lithium borate substrate obtained by polishing.

[0057]

[Table 5]

The peripheral edge of the polished lithium borate substrate was photographed using a scanning electron microscope according to the method described above, and the distance A and the distance B were measured. It was 3 μm.

Comparative Example 2 A suede-type polishing pad (SURFIN 018-3, manufactured by Fujimi Incorporated) was attached to a rotating platen (diameter 360 mm) of a high-speed lens polishing apparatus, and the platen was polished to a rotation speed of 100 rpm. Under a predetermined processing pressure of the material, a commercially available colloidal silica (COMPOL80, manufactured by Fujimi Incorporated) was prepared so as to have a silica (silicon dioxide) content of 20% by weight using a lithium borate substrate as the material to be polished. Polishing liquid (liquid temperature: 2
Using 5 ° C., pH = 12), the polishing liquid was dropped at a rate of 100 ml / min and polished while circulating. Table 5 shows the measurement results of the surface accuracy as the obtained results, and FIG. 5 shows a sketch of an atomic force microscope photograph of the surface of the lithium borate substrate obtained by polishing.

Further, the periphery of the polished lithium borate substrate was photographed using a scanning electron microscope according to the method described above, and the distance A and the distance B were measured. The distance A and the distance B were about 200 μm and about 100 μm, respectively. there were.

As described above, the surface roughness and the surface roughness of the embodiment are smaller than those of the comparative example, so that the substrate surface can be polished smoothly, and the peripheral portion of the material to be polished is also polished during the polishing. It turns out that there is almost nothing.

[Brief description of the drawings]

FIG. 1 is an example of a cross-sectional view (conceptual diagram) of a part of a substrate showing surface droop in a polished substrate for calculating the amount of surface droop.

FIG. 2 is an atomic force microscope sketch of the surface of a lithium tantalate substrate obtained by a polishing test in Example 1.

FIG. 3 is an atomic force microscope sketch of the surface of a lithium tantalate substrate obtained by a polishing test of Comparative Example 1.

4 is an atomic force microscope sketch of the surface of a lithium borate substrate obtained by a polishing test in Example 2. FIG.

5 is an atomic force microscope sketch of the surface of a lithium borate substrate obtained by a polishing test of Comparative Example 2. FIG.

[Explanation of symbols]

1: Line extrapolated from the tangent line of the polished surface of the substrate 2: Boundary between the surface drooping portion on the substrate surface and the portion where no surface drooping occurs on the inside surface 3: Right angle to the substrate surface from position 2 4: a line extrapolating a tangent in the thickness direction of the substrate, which intersects at right angles with the line 1 Boundary with the part 6: Line drawn perpendicular to the thickness direction of the substrate from the position of 5 A: Range where surface drooping occurs at the periphery of the substrate surface (distance between lines 3 and 4) B: Substrate of the substrate Surface sagging in the thickness direction (distance between line 1 and line 6)

The single crystal substrate of the present invention has better surface roughness than the conventional one, has a very small surface droop, and has a flat surface. Alternatively, it is useful as a material for a surface acoustic wave filter. According to the method for manufacturing a single crystal substrate according to the present invention, a single crystal substrate having such excellent characteristics can be easily manufactured.

Claims (3)

[Claims] 1. The substrate has a center line average roughness of 0.3 nm or less, and a surface droop of the substrate is 20 μm from a peripheral edge of the substrate.
Within 1 μm, and wholly in the thickness direction of the substrate
m or less. 2. The single crystal substrate according to claim 1, wherein the single crystal substrate is a single crystal substrate of lithium tantalate or lithium borate. 3. A method for producing a lithium borate single crystal substrate by cutting a single crystal ingot to obtain a plate-like wafer, polishing the wafer to a desired thickness by double-sided lap, and polishing the mirror to a mirror surface. , Mainly silica (silicon dioxide)
, And having a bulk density of 0.2 to 1.5 g / cm ³ , a BET specific surface area of 10 to 400 m ² / g, and an average particle size of 0.001 to 0.5 μm. 3. The method for manufacturing a single crystal substrate according to claim 1, wherein the mirror polishing is performed by pressing the wafer against the surface and rubbing the wafer.